1. Field of the Invention
The present invention relates to an image forming apparatus. More particularly, the present invention relates to an image forming apparatus which forms visual images using a two-component developer including a toner and a carrier.
2. Discussion of the Background
One of background developing devices using a two-component developer including a toner and a magnetic carrier is illustrated in FIG. 1. Referring to FIG. 1, a background developing device 4a has two separated developer passages, i.e., a first developer passage for supplying a developer to a developing roller 5 (serving as a developer bearing member) and a second developer passage 10 (i.e., a developer agitating passage) for agitating the developer. The developer in the first developer passage is fed in a direction opposite to the feeding direction of the developer in the second developer passage 10 so that the developer is circulated in the two developer passages. In FIG. 1, numerals 401, 403 and 11 denote a first auger for feeding the developer, a partition, and a second auger.
In the background developing device 4a illustrated in FIG. 1, the first developer passage for supplying a developer to the developing roller 5 also serves as a developer collecting passage for collecting the developer passing through a development region while being used for developing electrostatic latent images on an image bearing member. Therefore, the concentration of toner in the developer decreases in the developer feeding direction in the first developer passage. Namely, the developer on the downstream side in the first developer passage relative to the developer feeding direction has lower toner concentration than the developer on the upstream side. Therefore, a problem in that images having uneven image density are formed occurs.
In attempting to avoid such an uneven density image problem, published unexamined Japanese patent applications Nos. (hereinafter referred to as JP-As) 06-51634 and 11-167260 have proposed developing devices in which a developer supplying auger and a developer collecting auger for collecting the developer used for developing are arranged in different developer passages. Hereinafter, each of the background developing devices will be explained in detail.
The background developing device proposed by JP-A 06-51634 is illustrated in FIG. 2. Referring to FIG. 2, a background developing device 4b includes a developer supplying passage 9 for supplying a developer to a developing roller 5, and a developer collecting passage 7 for collecting the developer passing through a development region at which the developing roller 5 is opposed to an electrostatic latent image bearing member 1, wherein the developer collecting passage 7 is separated from the developer supplying passage 9. Since the developer passing through the development region is fed to the developer collecting passage 7, the developer is not mixed with the developer in the developer supplying passage 9. Therefore, the toner concentration of the developer in the developer supplying passage 9 (i.e., the toner concentration of the developer fed to the developing roller 5) hardly changes.
However, the collected developer fed to the developer collecting passage 7 is shortly supplied to the developer supplying passage 9 after a fresh toner is supplied to the collected developer (this developer is hereinafter sometimes referred to as a recovered developer). Therefore, even when the recovered developer has a proper toner concentration, problems in that uneven density images or low density images are produced occur. This is because the recovered developer (i.e., the mixture of the collected developer and the fresh toner) is not sufficiently agitated. The problems are remarkably caused when the collected developer has been used for developing images having a high image area proportion. In FIG. 2, numerals 8, 6 and 209 denote first, second and third augers, and numerals 15c and 16R denote a center of the developing roller 5 and a developer thickness controlling member for controlling the thickness of the developer on the developing roller 5.
The background developing device proposed by JP-A 11-167260 is illustrated in FIG. 3. In a background developing device 4c illustrated in FIG. 3, a developer supplying passage 9 for supplying a developer to a developing roller 5 is separated from a developer collecting passage 7 for collecting the developer passing through the development region at which the developing roller 5 is opposed to an electrostatic latent image bearing member 1. The developing device 4c further includes a developer agitating passage 10 for agitating the developer, which is fed to the downmost stream side of the developer supplying passage 9, and the collected developer, which is fed to the downmost stream side of the developer collecting passage 7, while feeding the mixed developer in the direction opposite to the developer feeding direction in the developer supplying passage 9.
In the developing device 4c, the collected developer is fed to the developer collecting passage 7, and therefore the collected developer is not mixed with the developer in the developer supplying passage 9. Therefore, the toner concentration of the developer in the developer supplying passage 9 (i.e., the toner concentration of the developer fed to the developing roller 5) hardly changes.
In the developing device 4c, the collected developer is mixed with the developer fed through the developer supplying passage 9 without being used for development, followed by agitating in the developer agitating passage 10. The thus mixed developer is supplied to the developer supplying passage 9. Therefore, the above-mentioned problems in that uneven density images or low density images are produced are hardly caused.
However, in the developing device 4c, the developer is not directly returned from the development region to developer supplying passage 9. Therefore, in order to stably feed the developer toward the downstream side of the developer supplying passage 9, the speed of feeding the developer through the developer supplying passage 9 has to be faster than the speed of feeding the developer to the development region. In this case, a high stress is applied to the developer in the developer supplying passage 9, resulting in acceleration of deterioration of the carrier in the developer, thereby shortening the life of the developer. In FIG. 3, numerals 404 and 405 denote partitions, and numeral 27c denotes a toner concentration sensor for detecting the toner concentration of the developer.
In developing operations of general developing device 5 using a two component developer, the toner is consumed while the carrier is not consumed and stays in the developing device. Therefore, the carrier, which is agitated together with the toner in the developing device, deteriorates as the frequency of agitation (or agitation time) increases. This is because the resin layer formed on the surface of the carrier is peeled or the toner is adhered to the resin layer, resulting in deterioration of the resistivity of the carrier and charging property of the developer. In this case, the developing property of the developer changes (so as to be excessively enhanced), thereby causing problems in that the image density increases and the background of images is developed with the toner (i.e., background development occurs).
In attempting to solve the problem, JP-A59-100471 proposes a trickle developing method in that a mixture of a toner and a carrier is added to compensate the toner used for development while replacing the carrier little by little in the developing device. However, even in such a developing device, the amount of deteriorated carrier particles increases after long repeated use. Therefore, it is difficult for the developing device to prevent occurrence of the problems in that the image density increases and the background of images is developed with the toner.
JP-A 03-145678 discloses a technology in that a developer supplement, which includes a toner and a carrier having a higher resistance than the carrier in the developing device, is supplied to the developing device in attempting to maintain the charging property of the developer, resulting in prevention of deterioration of image qualities.
In addition, JP-A 11-223960 (corresponding to U.S. Pat. No. 6,096,466) discloses a technology in that a developer supplement, which includes a toner and a carrier capable of imparting a larger amount of charge quantity to the toner than the carrier in the developing device, is supplied to the developing device in attempting to maintain the charging property of the developer, resulting in prevention of deterioration of image qualities.
However, the technologies proposed by JP-As 03-145678 and 11-223960 have a drawback in that the amount of the supplementary carrier replaced with the carrier in the developer changes depending on the amount of toner consumed for development, thereby changing the resistance and charging quantity of the developer in the developing device, resulting in variation of image density.
JP-A 08-234550 discloses a technology in that plural kinds of developer supplements, which are contained in a container while forming layers and each of which includes a toner and a carrier, wherein each of the carriers therein is different in property from the carrier in the developing device, are supplied to the developing device one by one. However, the technology has a drawback in that it is difficult to supply the plural kinds of developer supplements (contained in a container) without mixing the developer supplements. In addition, since the toner is contained in each of the developer supplements at a higher concentration than that in the developer in the developing device, the carrier tends to easily deteriorate. Therefore, it is hard for the developer to stably produce high quality images for a long period of time.
In addition, it is described in JP-A 08-234550 that the amount of silicone resin layer formed on the core particles of the supplementary carriers is increased to increase the resistance of the supplementary carriers. In this case, although the resistance of the coated carrier can be increased, the charging quantity of the carrier decreases, resulting in deterioration of reproducibility of the developed images and/or occurrence of the background development problem.
Therefore, it is necessary for the above-mentioned trickle development methods (i.e., the technologies using a developer supplement) that the carrier in the developer supplement can stably maintain a good charge imparting property even when used for a long period of time, in order that the developer in the developing device maintains a good developing property.
Two component developers typically use a coated carrier to prevent formation of a film of toner on the carrier; to form a uniform surface on the carrier; to prevent the surface of the carrier from being oxidized; to improve the humidity resistance of the carrier; to prolong the life of the developer; to prevent the image bearing member (such as photoreceptors) from being scratched or abraded by the carrier; and to control the charging properties (such as polarity and charge quantity). For example, JP-A 58-108548 discloses a carrier covered with a resin, and JP-As 57-168255, 58-117555 and 06-202381 have disclosed carriers covered with a resin layer including an additive.
JP-A 05-273789 discloses a technology in that an additive is adhered to the surface of a carrier. JP-A 09-160304 discloses a technology in that an electroconductive particulate material having a larger diameter than the thickness of the cover layer of the carrier is included in the cover layer.
In addition, JP-A 08-6307 discloses to use a benzoguanamine/n-butylalcohol/formaldehyde copolymer for the cover layer of the carrier. Further, JP-A 02-79862 discloses to use a crosslinked material of a melamine resin and an acrylic resin for the cover layer of the carrier.
These proposals for enhancing the durability of the cover layer of the carrier are effective for the developer supplements mentioned above for use in the trickle developing methods because the developers can maintain a good charge imparting ability for a long period of time. However, the needs for durability of developer become severer and severer. Therefore, the needs for durability cannot be satisfied only by using such coated carriers. Specifically, occurrence of a spent toner problem in that the toner used is adhered to the surface of the carrier, deterioration of charging property of the developer due to the spent toner problem, and the background development problem cannot be fully solved by these technologies.
In addition, the trickle development methods have another drawback in that when a developer supplement including a toner and a carrier having a poor fluidity is supplied, the developer has a poor feeding property, resulting in occurrence of a feeding problem in that the developer is unevenly fed in the developing device.
Because of these reasons, a need exists for an image forming apparatus which hardly deteriorates the developer even when the developer feeding speed is increased, and hardly causes the above-mentioned problems even when a trickle development method is used.